Foucault’s early experiments in parabolizing glass led him to important results. By depositing a thin coating of silver on his specula he obtained a reflective power far surpassing that of metal. Thereafter metal was not thought of as a suitable material for reflecting-telescopes. Silver-on-glass mirrors immediately came into great request. The latter undoubtedly possess a great superiority over metal, especially as regards light-grasping power, the relative capacity according to Sir J. Herschel being as ·824 to ·436. Glass mirrors have also another advantage in being less heavy than those of metal. It is true the silver film is not very durable, but it can be renewed at any time with little trouble or expense. With of Hereford, and after him Calver of Chelmsford, became noted for the excellency of their glass mirrors. They were found nearly comparable to refractors of the same aperture. A tendency of the times was evidently in the direction of large instruments. One of 47·2-inches aperture (for which a sum of 190,000 francs was paid) was completed by Martin in 1875 for the Paris Observatory, but its employment since that year has not furnished a very successful record. The largest instrument of the kind yet made has a speculum 5 feet in diameter and 27½-feet focal length. It was placed in position in September 1888, and was made by the owner, Mr. Common, of Ealing, whose previous instrument was a 37-inch glass reflector by Calver. The 5-foot telescope is undoubtedly of much greater capacity than the colossal reflector of Lord Rosse, though it is not so large. Mr. Calver has recently figured a 50-inch mirror for Sir H. Bessemer, but the mounting is not completed; and he is expecting to make other large reflectors, viz. one over 5 feet in diameter and another over 3 feet. The late Mr. Nasmyth also erected some fine instruments, and adopted a combination of the Cassegrainian and Newtonian forms to ensure greater convenience for the observer. Instead of permitting the rays from the small convex mirror to return through the large mirror, he diverted them through the side of the tube by means of a flat mirror, as in Newtonians. But this construction is not to be commended, because much light is lost and defects increased by the additional mirror. Smaller telescopes of the kind we have been referring to have become extremely popular: and deservedly so. They are likely to maintain their character in future years; for the Newtonian form of instrument, besides being thoroughly effective in critical work, is moderate in price and gives images absolutely achromatic. Moreover, it is used with a facility and ease which an experienced observer knows how to appreciate. Whatever may be the altitude of the objects under scrutiny, he is enabled to retain a perfectly convenient and natural posture, and may pursue his work during long intervals without any of the fatigue or discomfort incidental to the use of certain other forms of instrument.